Wait one moment while I put on my pedantic hat. There.
OK, class, let's review:
1. A coil is a way to get high voltage out of low voltage. It is, in fact, a
transformer with a primary (low voltage) and secondary (high voltage) COIL. It
is the change in magnetic field in the primary which induces a proportionately
higher voltage in the secondary. In order for there to be a change in field,
the current must alternate (AC) or be interrupted (pulsating DC). Remember, if
we run DC through the coil continuously (ignition on, points closed, car not
running) we turn the coil into a heater which cooks itself.
2. The points control the interruption of flow of DC to the primary of the
coil. As the points open and close the voltage induced in the secondary will
flow wherever it can; or nowhere if not connected to a conductor; or jump an
air gap if not too large to overcome the difference in potential, say across
the space of a correctly gapped spark plug. Remember, it is the points closing
and opening that give us that burst of energy from the secondary of the coil.
3. The rotor spinning around inside the distributor simply provides an
appropriate path for the high voltage to go to each spark plug. Why? Because we
want to fire more than one spark plug. That is why it is a *distributor*. Now,
listen up; this is the important part. The path for the high voltage from the
coil, through the tower of the distributor cap, through the rotor, through the
spark plug contact inside the cap, through the spark plug wire, and ultimately
to the spark plug is already established at the time the points are open and
close and open again. The path better already be there or the voltage is
'wasted' - doesn't go anywhere. Also, high voltage would be trying to arc from
the rotor to the nearest spark plug contact if the field were collapsing while
the connection is not complete. Not desirable. At no time is the rotor holding
back some pent up charge. The points make the coil do all the boom boom stuff.
Extra credit: The spark plug contacts in the cap are indexed to the rest of the
mechanism so that the rotor is in closed contact with a spark plug when the
coil 'fires'. The only way for it to be out of registry is if the distributor
shaft is twisted or something. Rotating this whole mess a few degrees one way
or the other changes when all this happens in relation to the position of the
pistons in the engine (timing and advance).
Pedantic hat off. I made this explanation all up myself, so tell me where I am
wrong. I think Chris here (below) has got it the 'most right'.
Cheers,
Jon Larsen (79 MGB)
>>> Christopher Palmer <ctp@gbn.org> 12/17 7:49 PM >>>
In that it holds energy yes, except that it holds it inductively not
capacitively. And yes, the rotor helps provide that path to ground. The
points make/break so the coil can charge.
Points closed, coil charges...points open, spark to ground thru plug.
CTP
>Actually, as I understand it (please correct me if I'm wrong) the coil
>is in essence a large capacitor, whose fucntion is to hold a charge
>until such time as there is a path to ground. So the charge would in
>fact sit ( in the capacitor, not the rotor though) until the rotor came
>around and made contact with cap, wich provides the charge in the
>capacitor (ignition coil) with a path to ground ( via the spark plug).
>
>Boom, plug fires.
>
>Greg
At 5:34 PM -0800 12/17/97, Gregory Kirk wrote:
>Trevor Boicey wrote:
>> > This happens as soon as the rotor swings around to an electrode inside
>> > the cap. Boom! A sparkplug ignites.
>>
>> That's simply not true. The voltage will not "sit" on the rotor
>> waiting for the proper gap before firing.
>>
>> The voltage appears at the rotor when produced by the coil. It
>> will either arc to the metal if the metal is close, or nothing
>> will happen. It will not "sit and wait" and screw up your timing,
>> that's simply not possible.
>>
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